Many Thanks "Fluffy Chicken", FULL MARKS!
I found what I needed on the Official Shimano Service leafelet you pointed to in the gallery.
http://www.retrobike.co.uk/gallery2/mai ... ewsIndex=3
It seems there are 3 default settings (not 4 as I thought), and the strongest is with the tiny arrow pointing to the punch mark, so I'll back-pedal it two clicks to set it to the weakest (least tension).
When I removed and cleaned my thumb shifters with paraffin, they work fluidly, with none of the excessive tension I experience when connected up.
So it must be downstream.
I'm happy now that I know how to get the RD tension set to the lowest.
Next I'll double check the cables to make sure I get the easiest up-changing. It +is possible+ a constriction on one of the outer cables is pinching the inner. Just leaning a loaded bike clumsily against a low wall can pinch it.
Thank you very much for your help.
The gallery of Service leaflets is invaluable. I never knew it was there, and many times I've searched for such data on the web.
Any chance you could post the one for the FD651 that's on my 1992 Panasonic? That's front deraileur.
Fascinating to see the little tool they illustrate to set up cantilever brake cables properly.:
http://www.retrobike.co.uk/gallery2/mai ... ewsIndex=1